Semiconductor memory cell and method for producing the same

ABSTRACT

The present invention provides a semiconductor memory cell having a semiconductor substrate ( 1 ); a trench ( 5 ) provided in the semiconductor substrate ( 1 ); a floating gate electrode ( 45 ) introduced in the trench ( 5 ), which electrode is insulated from the trench walls by a first insulation layer ( 50 ); a control gate electrode ( 80 ) provided in the semiconductor substrate ( 1 ) around the trench ( 5 ); a second insulation layer ( 10 ) provided on the surface of the semiconductor substrate ( 1 ); a conductive layer ( 20 ) provided on the second insulation layer ( 10 ), which conductive layer forms a channel region ( 35 ) above the floating gate electrode ( 45 ); and a source region ( 30 ) and drain region ( 40 ) formed in the conductive layer ( 20 ) in each case beside the channel region ( 35 ). The invention also provides a corresponding fabrication method.

[0001] The present invention relates to a semiconductor memory cell and a method for fabricating it.

[0002] Although applicable, in principle, to any desired semiconductor memory cells, the present invention and the problem area on which it is based are explained with regard to nonvolatile semiconductor memory cells using silicon technology.

[0003]FIG. 2 shows a diagrammatic illustration of the construction of a known nonvolatile memory cell using silicon technology.

[0004] In FIG. 2, reference symbol 1 designates a silicon semiconductor substrate of the p-conduction type, into which an n⁺-type source region S and an n⁺-type drain region D are introduced. A channel region CH lies between the source region S and the drain region D. A tunnel oxide TO is provided above the channel region CH, a floating gate FG being situated in turn above said tunnel oxide. A coupling oxide KO is provided on the floating gate FG, a control gate CG being situated above said coupling oxide.

[0005] During operation of this known nonvolatile memory cell, charge carriers are moved for tunneling through the tunnel oxide TO by the application of an external voltage between the control gate CG and the channel or the source/drain region S, D. In this case, the voltage of the control gate CG is transmitted capacitively via the coupling oxide KO to the floating gate FG. The present charge state of the floating gate FG determines the threshold voltage of the transistor, which ultimately comprises the stored information.

[0006] Competitiveness with regard to costs and performance of nonvolatile memory arrangements can only be ensured by advancing miniaturization. At the present time, internal voltages of up to 19 V are required in order to program or erase the known nonvolatile cells, as illustrated, for example, in FIG. 2. In order to handle voltages of this order of magnitude, it is necessary to expend a relatively large amount of chip area for the insulation (e.g. triple wells, “channel stop” counter-implantations, withdrawn diffusion regions, laterally enlarged insulation) and the internal voltage generation (charge pumps).

[0007] Increasing miniaturizations of the technologies of nonvolatile memories (flash, EEPROM) can only be achieved by lowering the programming voltage. The latter in turn is dependent on the coupling capacitance between the floating gate FG and the programming line (word line) or the control gate CG. A maximum coupling capacitance is desirable in this connection.

[0008] Furthermore, in the case of present-day memory generations, an integration between RAM memories (Random Access Memories) and NVM memories (nonvolatile memories) has never been seriously considered before on account of the complexity of the fabrication processes. Mixed production of DRAM and NVM technologies on a common equipment plant would currently require huge outlays with regard to equipment conversion and would be accompanied by poor equipment capacity utilizations and an increased outlay for production planning and process support.

[0009] The object of the present invention is to specify an improved semiconductor memory cell and a corresponding fabrication method, the coupling capacitance being increased.

[0010] According to the invention, this object is achieved by means of the semiconductor memory cell specified in claim 1 and the fabrication method specified in claim 6.

[0011] The idea on which the present invention is based consists in forming a cell in which the gate structure is accommodated in the substrate.

[0012] A known filled deep trench of DRAM technology is taken in order to form the floating gate therein. This type of known trench is optimized with regard to the capacitance, so that the coupling factor or the voltage punchthrough from the control gate to the floating gate can be significantly increased and, consequently, with the same tunnel-relevant voltage between channel and floating gate, it is possible to reduce the external voltage between channel and control gate.

[0013] This considerably reduces the amounts of area expended for lateral insulation. Moreover, the control gate can preferably be realized by simple doping reversal of the basic material of the semiconductor substrate (e.g. silicon).

[0014] A further advantage of the memory cell according to the invention and of the corresponding fabrication method is that mixed production as needed of DRAM and NVM technologies on a common equipment plant with minimal process support outlay is made possible. It thus becomes possible to integrate DRAM, logic and NVM technologies in one module. This additionally results in a minimization of the development outlay for subsequent shrink generations.

[0015] The fact that the control gate encloses the floating gate virtually completely, i.e. with the exception of the channel region at the surface, in the case of the memory cell according to the invention has the consequence that the coupling capacitance is maximized and the voltage requirement therefore decreases. New process steps are not required for fabricating the memory cells according to the invention, since, in particular, the formation of trenches or deep trenches is already well known from DRAM production.

[0016] The subclaims contain advantageous developments and improvements of the semiconductor memory cell specified in claim 1 and of the fabrication method specified in claim 6.

[0017] In accordance with one preferred development, the control gate electrode is a doping region in the semiconductor substrate.

[0018] In accordance with a further preferred development, the floating gate electrode is a polysilicon filling of the trench.

[0019] In accordance with a further preferred development, the conductive layer is an epitaxial layer made of polysilicon.

[0020] In accordance with a further preferred development, STI trenches for insulation with respect to adjacent memory cells are provided beside the trench.

[0021] An exemplary embodiment of the invention is illustrated in the drawings and is explained in more detail in the description below.

[0022] In the figures:

[0023]FIG. 1 shows a diagrammatic illustration of a semiconductor memory cell in accordance with a first embodiment of the present invention; and

[0024]FIG. 2 shows a diagrammatic illustration of a known semiconductor memory cell.

[0025] In the figures, identical reference symbols designate identical or functionally identical constituent parts.

[0026]FIG. 1 shows a diagrammatic illustration of a semiconductor memory cell in accordance with a first embodiment of the present invention.

[0027] In FIG. 1, reference symbol 1 designates a silicon semiconductor substrate of the p-conduction type. A trench 5 is provided in the semiconductor substrate 1, an SiO₂ insulation layer 50 being situated on the trench wall of said trench. The first insulation layer 50 serves as the coupling oxide known per se. A floating gate electrode 45 made of polysilicon is introduced into the trench 5 and isolated from the trench wall by said insulation layer 50.

[0028] An n⁺-type region serving as control gate electrode 80 is situated in the semiconductor substrate 1 around the trench 5. The connection of the control gate electrode 80 is not visible in FIG. 1 and, in this example, is routed toward to the rear from the plane of the drawing and at another location to the surface of the semiconductor substrate 1.

[0029] STI insulation regions 60, 70 STI=(Shallow Trench Isolation) are situated to the left and right of the trench 5 configured in this way.

[0030] Provided on the surface of the semiconductor substrate 1 is a second insulation layer 10 made of SiO₂ which serves as tunnel oxide. Provided on the second insulation layer 10 is a conductive layer 20 made of silicon, which forms a channel region 35 above the floating gate electrode 45. An n⁺-type source region 30 and an n⁺-type drain region 40 are situated beside the channel region 35.

[0031] The memory cell in accordance with this embodiment has the same functionality as the known memory cell in accordance with FIG. 2. However, its construction is virtually upside down, because the gate structure with the floating gate 45 and the control gate 80 is arranged below the channel region 35 in the semiconductor substrate 1.

[0032] A fabrication method for fabricating the memory cell shown in FIG. 1 is described below.

[0033] Firstly, the semiconductor substrate 1 of the p-conduction type is provided. This semiconductor substrate 1 may be a wafer substrate or some other substrate, for example an SOI substrate or an epitaxial substrate. According to known DRAM technology, the trench 5 is formed by an etching process in the substrate 1. The side walls of the trench 5 are oxidized in order to form the first insulation layer 50 and this is followed by suitable implantation or diffusion in order to form the control gate 80 as n⁺-type region in the semiconductor substrate 1 around the trench 5. Afterward, n⁺-doped polysilicon is deposited and planarized in order to form the floating gate 45 as trench filling. The STI trenches 60, 70 are formed to the left and right of the trench 5 according to known etching and filling technology.

[0034] In a further step, the second insulation layer 10, i.e. the tunnel oxide, is formed by thermal oxidation. An epitaxy step is preferably used to form the silicon layer 20 above the second insulation layer 10. There follows a masking and an introduction of n-type dopant for forming the source/drain region 30, 40.

[0035] As is evident, the fabrication of DRAMs and NVMs can be carried out simultaneously with this new fabrication method.

[0036] An example of application of such a process is the fabrication of hard-disk controllers. For these components, embedded DRAM modules are used for control and cache functions, whereas the control software is stored on a separate EEPROM. The technology proposed would enable the integration of this function on one module.

[0037] A further example of application is miniature computers (hand-held/notebooks). The latter are usually fabricated in embedded DRAM chips. With integrated technologies, the miniaturization can be advanced further here by incorporation of nonvolatile memories. Moreover, irksome booting is obviated, which creates time and energy advantages. The operating system and application programs, which have hitherto been provided on the hard disk, could be situated in the additionally integrated nonvolatile memory.

[0038] Although the present invention has been described above with reference to preferred exemplary embodiments, it is not restricted thereto but rather can be modified in diverse ways.

[0039] Although the present invention has been explained with reference to a nonvolatile memory cell using silicon technology, the invention is not restricted to silicon as basic material. Moreover, the other materials and method steps in the above exemplary embodiment are only specific possibilities from the many possibilities for realizing the memory cell according to the invention. 

1. Semiconductor memory cell having: a semiconductor substrate (1); a trench (5) provided in the semiconductor substrate (1); a floating gate electrode (45) introduced in the trench (5), which electrode is insulated from the trench walls by a first insulation layer (50); a control gate electrode (80) provided in the semiconductor substrate (1) around the trench (5); a second insulation layer (10) provided on the surface of the semiconductor substrate (1); a conductive layer (20) provided on the second insulation layer (10), which conductive layer forms a channel region (35) above the floating gate electrode (45); and a source region (30) and drain region (40) formed in the conductive layer (20) in each case beside the channel region (35).
 2. Semiconductor memory cell according to claim 1, characterized in that the control gate electrode (80) is a doping region in the semiconductor substrate (1).
 3. Semiconductor memory cell according to claim 1 or 2, characterized in that the floating gate electrode (45) is a polysilicon filling of the trench (5).
 4. Semiconductor memory cell according to claim 1, 2 or 3, characterized in that the conductive layer (20) is an epitaxial layer made of polysilicon.
 5. Semiconductor memory cell according to one of the preceding claims, characterized in that STI trenches (60, 70) for insulation with respect to adjacent memory cells are provided beside the trench (5).
 6. Method for fabricating a semiconductor memory cell having the steps of: providing a semiconductor substrate (1); forming a trench (5) in the semiconductor substrate (1); forming a first insulation layer (50) on the trench walls; forming a floating gate electrode (45) in the trench (5), which electrode is insulated from the trench walls by the first insulation layer (50); forming a control gate electrode (80) in the semiconductor substrate (1) around the trench (5); providing a second insulation layer (10) on the surface of the semiconductor substrate (1); providing a conductive layer (20) on the second insulation layer (10), which conductive layer forms a channel region (35) above the floating gate electrode (45); and forming a source region (30) and a drain region (40) in the conductive layer (20) in each case beside the channel region (35).
 7. Method according to claim 6, characterized in that the control gate electrode (80) is formed by doping reversal of the semiconductor substrate (1).
 8. Method according to claim 6 or 7, characterized in that the the [sic] floating gate electrode (45) is formed by filling the trench (5).
 9. Method according to claim 6, 7 or 8, characterized in that the conductive layer (20) is formed as an epitaxial layer.
 10. Method according to one of claims 6 to 9, characterized in that STI trenches (60, 70) for insulation with respect to adjacent memory cells are provided beside the trench (5). 